Trunked radio communication systems are well known in the communication system art. In such systems, groups of communication devices such as portable radios and mobiles communicate with each other over a group of radio frequencies (channels). Typically, trunked communication systems have a specific number of voice/data communication channels which are dynamically allocated between different groups of radios in the system by a system central controller. In a trunked communication system such as the Motorola SMARTNET.TM. system, the system central controller communicates over a control channel to all of the radios in the system in order to coordinate the activities of all the radios. The control channel in a trunked system is normally one of the radio frequency channels (repeaters) which has been designated to be the control channel. The messages transmitted over the control channel by the system central controller include: messages directing a specific group of radios to a specific system voice channel (repeater), messages from the radios in the system to the system central controller requesting the controller to assign them a voice channel, system identification information from the central controller to the radios, and other related information.
In trunked systems where protection of the voice/data messages is important, a system like the Motorola Digital Voice Protection System can be added to the individual radios in the system. The Digital Voice Protection System is an encryption system that converts an analog communication signal into a digital bit stream and then scrambles (encrypts) the digital bit stream with a technique to which only the sender and authorized receivers are given "keys" which allow the signal to be decrypted. If an eavesdropper tries to listen to conversations on this type of system, the encrypted messages will be able to be received but the messages will sound like pseudorandom noise unless the eavesdropper has the proper key to decrypt the message. Unfortunately, in present day trunked systems only the voice communications between radios on the system are encrypted (i.e. voice channel traffic). Data transmissions containing control and status information are not encrypted.
Since trunked systems like the SMARTNET.TM. system transmit the majority of the system control information via the control channel, an eavesdropper can gain valuable information as to the amount of traffic, the number of radios on the system, channel assignments, and other related information since the control channel is not encrypted in present day trunked communication systems. One of the major reasons why the trunked control channel is not encrypted is due to the fact that different groups of radios on a trunked system may be using different encryption algorithms and/or encryption keys at the same time. For example, radio group "A" may be using one encryption key for their communications, while group "B" may be using another encryption key, thereby not allowing for the control channel signals to be encrypted due to the encryption key incompatibility of the different radio groups. The problem is even more pronounced when different radio groups on a system are using different encryption algorithms (i.e. DVP-XL, DES-XL).
A need exists for a system and radios which can operate in a system where the control information carried by the control channel is also encrypted in order to protect the entire integrity of the trunked communication system from unwanted eavesdropping.